Crashes often under XFCE and Fvwm

[jgeiss66]

Version 0.4.6 crashes quite often with message
Plugin Window Header Title unexpectedly left the panel, do you want to restart it

Debugging it leads to a NULL pointer in windowck-title.c, line 108 (top_window == NULL).

Maybe changing the line to
while (top_window && top_window->data != window && top_window != bottom_window) {
fixes the bug. At least it worked on my computer so far.